Reductive Coupling of Aromatic Carbonyl Compoundsto Pinacols Using Zinc Powder in Aqueous Media$Lei Wang, Xinghua Sun and Yongmin Zhang*Department of Chemistry, Hangzhou University, Hangzhou, 310028, P.R. ChinaZinc-mediated reductive coupling of aromatic carbonyl compounds occurs to give corresponding 1,2-diols in moderateto good yields in saturated NH4Cl(aq)¡ÓTHF solution at room temperature.1,2-Diols are very useful synthons for a variety of organicsyntheses.1 The reductive coupling of carbonyl compoundsis an important method for the formation of 1,2-diols,2 andis usually carried out under anhydrous conditions.On theother hand, many reagents such as TiCl3,3 Zn¡ÓZnCl2,4Al(Hg),5 Cp2TiCl6 and Mn7 have been developed forpinacolic coupling reactions in aqueous media becauseorganic reactions in aqueous media oer a number ofadvantages over conventional organometallic reactions inorganic solvents.8 As a cheap and ecient reagent, metalliczinc powder has been used for the preparation of homo-allylic alcohols by the coupling of allylic halides with car-bonyl compounds in aqueous media.9 To the best of ourknowledge there are no literature examples of pinacoliccoupling using zinc powder in saturated NH4Cl(aq)¡ÓTHFsolution.We therefore report herein that zinc-mediatedreductive coupling of aromatic carbonyl compounds topinacols can be carried out in aqueous media at room tem-perature. The results are summarized in Table 1.Table 1 shows that aromatic aldehydes undergo couplingin the presence of zinc powder in THF¡ÓNH4Cl(aq) solutionto give pinacolic coupling products (1,2-diols) in moderateto good yields at ambient temperature. Except for that of2 g, the DL : meso ratio of diols (entries a¡Óf, h) is nearly 1:1(1H NMR).The reason for the eect of the triuoromethylgroup in the 4-position of the aromatic ring (entry g) on theDL:meso ratio is not clear. Unfortunately, the aromaticketone (entry h) gives the pinacolic coupling product inpoor yield.We have tried to use anhydrous THF, THF¡Ówater (4 :1)or water in place of THF¡ÓNH4Cl(aq) (4 :1) respectively.However, no product was formed under these conditions.Also, other metal powders such as tin and indium in placeof zinc gives no reaction.In summary, zinc is a useful metal to mediate the reduc-tive coupling of aromatic carbonyl compounds in aqueousmedia at room temperature. The advantages of this reactionare mild and neutral reaction conditions, simple operationand good yields.Experimental1H NMR spectra were recorded on a JEOL PMX 60 SIinstrument.All NMR samples were measured in CDCl3 using TMSas internal standard. IR spectra were obtained on a Perkin-Elmer683 spectrophotometer as KBr plates.General Procedure for the Preparation of Pinacols.The aromaticcarbonyl compound (1.0 mmol) and metallic zinc powder(1.2 mmol) were added to saturated aqueous NH4Cl¡ÓTHF (1:4,5 ml) solution. The resulting mixture was stirred at room tempera-ture for 8 h, then dilute hydrochloric acid (0.5 mol l£¾1, 2ml) wasadded to quench the reaction and the mixture was extracted withdiethyl ether (20 ml2).The extracts were washed with brine,J. Chem. Research (S),1998, 336¡Ó337$Table 1 Zinc-mediated reductive coupling of aromatic carbonyl compoundsEntry Ar R Yield(%)aDL : mesob 1H NMR(d) IR(~/cm£¾1)a C6H5 H 65 55 : 45 3.78 (2 H, s, OH),c 4.40 (s, DL) and 4.60 (s, meso)(2 H, 2 PhCH), 6.50¡Ó7.30 (10 H, m, Ph)3100¡Ó3600(s)b p-ClC6H4 H 77 58 : 42 2.85 (2 H, s, OH),c 4.40 (s, DL) and 4.63 (s, meso)(2 H, 2 PhCH), 6.60¡Ó7.30 (8 H, m, Ar)3200¡Ó3600(s)c p-BrC6H4 H 76 50 : 50 2.46 (2 H, s, OH),c 4.50 (s, DL) and 4.60 (s, meso)(2 H, 2 PhCH), 6.60¡Ó7.60 (8 H, m, Ar)3100¡Ó3600(s)d p-FC6H4 H 73 55 : 45 3.15 (2 H, s, OH),c 4.55 (s, DL) and 4.70 (s, meso)(2 H, 2 PhCH), 6.60¡Ó7.50 (8 H, m, Ar)3150¡Ó3620(s)e m-BrC6H4 H 70 52 : 48 3.33 (2 H, s, OH),c 4.33 (s, DL) and 4.50 (s, meso)(2 H, 2 PhCH), 6.70¡Ó7.40 (8 H, m, Ar)3100¡Ó3500(s)f o-BrC6H4 H 68 50 : 50 2.90 (2 H, s, OH),c 4.55 (s, DL) and 4.65 (s, meso)(2 H, 2 PhCH), 6.70¡Ó7.50 (8 H, m, Ar)3100¡Ó3600(s)g p-CF3C6H4 H 82 74 : 26 3.04 (2 H, s, OH),c 4.45 (s, DL) and 4.66 (s, meso)(2 H, 2 PhCH), 6.65¡Ó7.50(8 H, m, Ar)3100¡Ó3600(s)h C6H5 CH3 32 51 : 49 1.42 (s, DL) and 1.55 (s, meso) (6 H, 2 CH3), 2.40(2 H, s, OH),c 6.80¡Ó7.30 (10 H, m, Ph)3100¡Ó3600(s)aIsolated yields.bRatios determined from the intensities of benzylic protons in 1H NMR spectra(entries a¡Óg), in which the protons ofthe DL isomer appeared at a higher magnetic field compared to that of the meso isomer, or from the intensities of methyl protons in1H NMR spectra(entry h), in which the methyl protons of the DL isomer appeared at higher magnetic field compared to that of the mesoisomer.10 cIn all examples this signal disappeared on adding D2O.dried over anhydrous Na2SO4 and the solvent was removed underreduced pressure.The residue was then puried by preparative TLCon silica gel with light petroleum¡Óether as the eluent to give theproduct.$This is a Short Paper as dened in the Instructions for Authors,Section 5.0 [see J.Chem. Research (S), 1998, Issue 1]; there is there-fore no corresponding material in J. Chem. Research (M).*To receive any correspondence.336 J. CHEM. RESEARCH (S), 1998Received, 20th January 1998; Accepted, 25th February 1998 Paper E/8/00478A References 1 A. Ghribi, A. Alexakis and J. F.Normant, Tetrahedron Lett., 1984, 3083. 2 B. E. Kahn and E. D. Rieke, Chem. Rev., 1988, 88, 733; J. M. Pons and M. Santelli, Tetrahedron, 1988, 44, 4295. 3 A. Clerici and O. Porta, Tetrahedron Lett., 1982, 23, 3517. 4 K. Tanaka, S. Kishigami and F. Toda, J. Org. Chem., 1990, 55, 2981. 5 M. Hulce and T. LaVaute, Tetrahedron Lett., 1988, 29, 525. 6 M. C. Barden and J. Schwartz, J. Am. Chem. Soc., 1996, 118, 5484. 7 C. J. Li, Y. Meng, X. H. Yi, J. Ma and T. H. Chan, J. Org. Chem., 1997, 62, 8632. 8 C. J. Li, Chem. Rev., 1993, 93, 2023; A. Lubineau, J. Auge and Y. Queneau, Synthesis, 1994, 741; C. J. Li, Tetrahedron, 1996, 52, 5643. 9 C. Petrier and J. L. Luche, J. Org. Chem., 1985, 50, 910; J. Einhorn and J. 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